Field of the Invention
[0001] The invention is a reactor (located between the exit of the boiler and the stack
or in the stack) which is used to reduce NO
x emissions from commercial and industrial boilers. The reactor system adds a reducing
agent to the exhaust of the boiler, then the exhaust and reducing agent mix are directed
through a specific type of selective catalytic reduction (hereinafter "SCR") catalyst.
Background
Boilers
[0002] Boilers are used in various industrial and commercial applications mainly for steam
generation purposes. Boilers in industrial usage include but not limited to usage
in food processing plants, printing, chemical processing plants, power generation,
and refining applications. Some commercial boiler applications include but are not
limited to hospitals, universities, large shopping malls, offices, and airports. Two
types of boilers are used in industrial and commercial applications for the purpose
of steam or hot water. They are fire tube boilers and water tube boilers. The present
invention may be used with either type of boiler.
[0003] While such boilers can burn any type of fossil fuel, natural gas has been an increasing
source of fuel. Although natural gas is several times cleaner than many other fuels,
combustion of natural gas inside such boilers results in the production of significant
amount of polluting emissions. Both NO and NO
2 are produced and are collectively termed "NO
x" or "nitrogen oxides." Several regulatory agencies in the US and across the world
have increased the stringent requirements for emission reductions from such industrial
and commercial boilers.
[0004] Another issue facing boiler usage is rising energy costs. It would be a great benefit
to find a way to reduce energy costs while increasing boiler efficiencies.
Burners
[0005] There are additional problems associated with the burners used with such boilers.
It is well known that conventional or standard burners used in boilers are highly
reliable; however they result in significant amount of NO
x production. A boiler with a standard burner typically results in 80-120 ppm of NO
x, whereas some environmental limits are below 30 ppm NO
x. In states such as California, NO
x emission limits from such industrial and commercial boilers are expected to be reduced
to below 5 ppm.
[0006] To solve the NO
x emission problem from boilers, boiler industry and users have moved from the use
of standard burners to low NO
x and ultra low NO
x burners. While low NO
x and ultra low NO
x burners allow NO
x reductions from burners to less than 30 ppm and below 9 ppm, they are very expensive.
Excess fuel is burned to achieve the same efficiency, thereby increasing the total
CO
2 content in the combusted exhaust. Moreover, these burners require frequent maintenance
and are not easy to operate. Boiler and Burner manufacturers and users have gradually
accepted the use of low NOx burners even though they are costly, since they result
in lower NOx emissions when compared to standard burners. This helps with compliance
with emission laws but at the expense of energy costs and lower boiler efficiencies.
Use of a low NOx burners results in about 30 - 60 ppm of NOx. As NOx regulations drive
NOx emissions even lower, in most cases below 30 ppm and some cases below 9 ppm, the
use of expensive ultra low NOx burners are being recommended. Table 1 presents comparison
of various features of different types of burners typically used in industrial and
commercial boilers.
Table 1: General comparison of various features for boilers using standard, Low NOx
and Ultra Low NOx burners
Feature |
Conventional or Standard Burners |
Low NOx burners |
Ultra Low NOx burners |
Flame Stability |
Highly stable |
Somewhat stable |
Unstable |
Reliability |
Highly reliable |
reliable |
Unreliable |
Boiler Turndown efficiency |
Excellent (> 10:1) |
Good to moderate (< 6:1) |
Poor (<4:1) |
Cost |
Low |
Moderate to high |
Expensive |
Excess O2 in flue gas |
1-3% |
3-5% |
3-8% |
NOx in flue gas |
80-120 ppm with no FGR |
30-60 ppm with 10% FGR |
< 30-9 ppm with 25-30% FGR |
Performance |
Nameplate efficiency |
Some loss of efficiency |
3-20% loss of nameplate efficiency |
FGR with burner |
None or little |
5-15% FGR |
10-35% FGR |
Maintenance |
Low |
Moderate |
High |
Steam ramp rate |
Fast |
Good steam ramp rate |
Poor steam ramp rate |
Thermal Efficiency |
High boiler thermal efficiency |
Some loss of thermal efficiency |
Poor boiler thermal efficiency |
[0007] Also problematic, low NO
x and ultra low NOx burners are typically associated with exhaust recirculation (Flue
Gas Recirculation hereinafter referred to as "FGR" or Exhaust Gas Recirculation "EGR")
of up to 30%. Exhaust is drawn inside by the use of fans, which consume additional
power. Typical power consumption can be in the range of 30-40% higher than standard
burners.
[0008] To sum up, the disadvantages of low and ultra low NO
x burners are: they are expensive, require more fuel, require a large excess of O
2 to achieve the same performance as a standard burner, have poor flame stability,
have higher FGR, produce more CO
2, and require more frequent maintenance.
SCR Catalysts
[0009] The present invention utilizes a zeolite based SCR catalyst (defined below) at the
back-end of the boiler to reduce the NO
x in the exhaust.
[0010] Zeolite based SCR catalysts are used in emission reductions for various stationary
power generating equipment such as gas turbines, but have not been used with standard
boilers. Most industrial boilers have economizers, to utilize as much heat as possible
which would otherwise be wasted. Therefore a SCR catalyst that can perform well at
low temperatures is essential for boiler applications. Typical exhaust temperatures
at the end of the boiler economizer is in the 480K (400°F) range, where as it could
be as high as 590K (600°F) before the economizer. Boiler stack temperatures are usually
around 370-450K (200-350°F).
[0011] Conventional SCR catalysts are based on vanadia and/or titania (hereinafter referred
to as "Conventional SCR Catalyst"). Imanari, (U.S. Patent 4,833,113), for example,
describes an SCR catalyst comprising an oxide of titanium, an oxide of tungsten, and
an oxide of vanadium. Conventional SCR catalysts such as the V/TiO
2 and the modified V/TiO
2 have their maximum performance in the temperature window of 530 to 700K (500 to 800°F)
and more preferably in the temperature window of 640 to 700K (700 to 800°F). However,
industrial and commercial boiler exhausts are a lot colder than that. Therefore there
is a strong need for a high performance SCR catalyst that can perform well at all
temperatures (420-590K (300-600°F)) depending on the boiler and economizer installation
and which can withstand the stringent boiler exhaust conditions.
[0012] Furthermore, the large excess of water vapor (20%), low temperatures of boiler exhaust,
and the presence of contaminants such as alkali metals, SO
2, etc. (as is known in the art), all degrade the performance of conventional catalysts
(including but not limited to conventional SCR catalysts) especially at low temperatures.
[0013] Byrne (U.S. Patent 4,961,917) discloses a method of passing ammonia, nitrogen oxides, and oxygen over iron or
copper-promoted zeolite catalysts to selectively catalyze the reduction of NO
x. These catalysts in Byrne and the conventional SCR catalysts are specifically excluded
from the zeolite based SCR catalysts as defined later in this specification. The fresh
copper-promoted catalyst has good activity. However, the copper catalyst deactivates
significantly when aged. Although the iron catalyst is far more stable than the copper
catalyst, it has maximum activity at about 622-773K (660-932°F), significantly higher
than the required 420-590K (300-600°F) temperatures that occur in industrial and commercial
boilers. There is a strong need for a SCR catalyst that can work well in the temperature
window of 420-590K (300 to 600°F).
[0014] Another feature of industrial and commercial boiler exhaust is that it contains about
10 to 20% water vapor and about 1 to 8% O
2. The amount of water vapor is higher for boilers that any other stationary sources
such as diesel engines, gas turbines, gas engines etc. Also the amount of O
2 is lower when compared to such stationary sources. This imposes difficulty on a Conventional
SCR
[0015] Catalyst to perform at such stringent conditions of temperature and exhaust compositions.
[0016] Another problem regarding the industrial and commercial boilers is their tum-down
feature. Depending on the steam production requirement, industrial and commercial
boilers can operate at either a maximum firing condition, at a minimum firing condition
or at any firing condition in between. The industrial and/or commercial boiler operation
is usually carried out at varying load conditions resulting in vast differences in
the temperature of firing conditions during a boiler operation. It is this temperature
swing in firing conditions that results in vast temperature differences in the boiler
exhaust. Industrial and commercial boilers can typically operate between about 590K
(600°F) at maximum firing conditions and below about 480K (400°F) at minimum firing
conditions. The amount of exhaust through the boiler also varies as the firing conditions
vary. Low firing conditions will result in lower exhaust from the boiler. Higher firing
conditions will result in larger amounts of exhaust gases from the boiler. Therefore
it is important for the catalyst to have good NO
x conversion activity over the entire range of boiler firing conditions. For a given
amount of catalyst volume in a SCR reactor in a boiler stack, this means that the
catalyst should operate at lower space velocities at low fire conditions and at higher
space velocities at high fire conditions. Space velocity is defined as the volume
of the exhaust through the SCR reactor in SCFH (standard cubic feet per hour) divided
by the volume of the catalyst in cubic feet. In such a calculation, the units for
space velocity are hr-1.
[0017] The installation of some types of SCR catalyst (but not zeolite based SCR catalyst)
behind industrial and commercial boilers for the purpose of NOx reduction has been
commercialized. CRI catalyst company, a division of Shell, presented a paper at the
3
rd International symposium on incineration and flue gas treatment technologies conducted
in July 2001 in Brussels wherein they describe their DeNOx system that contains SCR
catalyst for the purpose of NOx reduction. This reactor is located downstream of the
boiler, wherein the stack of the boiler is replaced or modified with a SCR reactor.
Ammonia is injected before the SCR reactor and the catalyst allows NOx reduction utilizing
the reducing agent. One disadvantage of this system is that the temperature in the
boiler stack seldom exceeds 480K (400°F). To achieve a great degree of NOx reduction,
significant amount of catalyst need to be used. Also when there is a lot of catalyst
being used, it creates excessive back pressure due to the presence of the large SCR
reactor. Back pressure in a fire tube or a water tube boiler seldom exceed 0.076 meters
(3 inches) of water column pressure. More importantly for fire tube boilers, it is
preferred to have back pressure losses around less than 0.025 meters (one inch) of
water column.
US2006/159607 relates to the abatement of nitrogen oxides and more particularly to the Selective
Catalytic Reduction (SCR) of nitrogen oxides using a zeolite catalyst impregnated
with iron, cerium, and manganese.
Summary of the Invention
[0018] The present invention provides a more economical, more reliable and energy efficient
way of removing NOx from boiler exhaust. The invention is a reactor system as defined
in claim 1 which combines a boiler with a back-end post combustion emission system
using a zeolite based SCR catalyst technology. This system comprises one or more inlets
for the introduction of one or more reducing agents downstream of the boiler. The
reducing agent comprises ammonia. As described herein, the reducing agent(s) can be
introduced into the exhaust from the boiler before or while the exhaust and reducing
agent mixture (hereinafter "Mixture') is directed through a zeolite based SCR catalyst.
The catalyst is optionally arranged in layers.
[0019] The zeolite based SCR catalyst of the present invention is defined as comprising:
(a) a first component selected from the group consisting of manganese and/or copper;
(b) a second component selected from the group consisting of cerium, a lanthanide,
a mixtures of lanthanides, and mixtures thereof; (c) a zeolite; and (d) a one oxygen
storage material, wherein the oxygen storage material is a cerium oxide-based material
comprising in addition one or more metal oxides and having a composition with a formula
Ce
1-aZr
aO
2 or Ce
1-c-dZr
cLan
dO
2, wherein Lan is selected from the group consisting of Y, La, Pr, Nd, Sm, Eu, Gd,
Ho, Yb and mixtures thereof. Lanthanides (or "Lan") are defined to mean Y, La, Pr,
Nd, Sm, Eu, Gd, Ho or Yb.
[0020] Optionally, embodiments may include one or more economizers. The zeolite based SCR
catalyst may be located in the economizer, upstream or downstream of the economizer.
[0021] Using the zeolite based SCR catalyst will allow the user to avoid high cost, high
maintenance low NO
x and/or ultra low NO
x burners. A standard burner can be used instead in combination with this zeolite based
SCR catalyst. Such a combination will result in reducing any amount of NO
x to almost negligible amounts, upon dosage of stoichiometric amounts of reducing agents,
e.g. without limitation ammonia. Not only are emission reductions achieved, but due
to the use of a standard burner in combination with this zeolite based SCR catalyst,
one can also achieve efficiency benefits, energy savings and cost savings. In situations
where the boiler already has a low NOx burner and is required to even further lower
NOx emissions than the low NOx burner can achieve, (below 30 ppm NOx for example),
the zeolite-based SCR catalyst will allow such NOx reductions in addition to efficiency
benefits, energy savings and cost savings compared to an ultra low NOx burner without
a zeolite based SCR catalyst. In situations where the boiler already has an ultra
low NOx burner generating 9 ppm NOx, and regulations force to be below the levels
attained by the ultra low NOx burners, the zeolite based SCR catalyst will allow such
NOx reductions.
[0022] A zeolite based SCR catalyst can work well in such low to medium temperatures (about
420-about 590K (about 300-about 600°F)) seen in the exhaust of such boilers using
reducing agents for the purpose of NOx reduction. This catalyst is capable of trimming
NOx levels to below 5 ppm levels, in the temperature range of about 420 - about 590K
(about 300-about 600°F) when the Mixture is directed through it after exiting from
a boiler.
[0023] The reducing agent comprises ammonia. Ammonia can be introduced into the exhaust
by any known method. Ammonia can be generated from anhydrous ammonia, aqueous ammonia,
ammonium hydroxide, ammonium formate, urea or any compound capable of generating ammonia
from it. Other known methods of forming ammonia in front of the zeolite based SCR
catalyst may also be used accordingly. For example without limitation, a hydrocarbon
(e.g. without limitation ethanol) in combination with a suitable catalyst can be used
upstream of the zeolite based catalyst for the purpose of generating ammonia for NOx
reduction. In such a situation, ammonia is generated by the reaction of the hydrocarbon
with the exhaust on the suitable catalyst site. A typical catalyst generating ammonia
from ethanol in a exhaust stream can be a silver based catalyst (See
US Patent 6,284,211).
[0024] In a typical boiler installation in the prior art, with the standard burner, there
is about 80-120 ppm NOx in the exhaust. In the prior art, this can be trimmed down
to 40-60 ppm NOx by the use of 10% FGR, unlike the case of an ultra low NOx burner
(where 30% FGR is required).
[0025] By using a standard burner, the invention largely eliminates all disadvantages associated
with low NOx and ultra low NOx burners set forth in the Background section. It is
also possible to reap certain benefits by choosing a low NO
x burner with a zeolite based SCR catalyst system in comparison to an ultra low NO
x burner without a zeolite based SCR catalyst system for NO
x reductions.
[0026] Installing a reactor with a zeolite-based SCR catalyst and introducing the reducing
agent before, in or after the economizer allows greater than 90% NOx conversion. Thus
NO
x can be trimmed down to single digit numbers.
[0027] The present invention selectively reduces NO
x in an exhaust by contacting the exhaust with a reducing agent and placing the Mixture
in the presence of a catalyst with a first component selected from the group consisting
of copper and/or manganese; a second component selected from the group consisting
of cerium, a lanthanide, a mixture of lanthanides, and mixtures thereof; at least
one oxygen storage material; and at least one zeolite, wherein the oxygen storage
material is a cerium oxide-based material comprising in addition one or more metal
oxides and having a composition with a formula Ce
1-a Zr
aO
2 or Ce
1-c-dZr
cLan
dO
2, wherein Lan is selected from the group consisting of Y, La, Pr, Nd, Sm, Eu, Gd,
Ho, Yb and mixtures thereof.
[0028] In one embodiment, a zeolite based SCR catalyst comprises a first component selected
from the group consisting of copper and/or manganese, ; a second component selected
from the group consisting of cerium, a lanthanide, a mixture of lanthanides, and mixtures
thereof; and at least one zeolite and at least one oxygen storage material, wherein
the oxygen storage material is a cerium oxide-based material comprising in addition
one or more metal oxides and having a composition with a formula Ce
1-a Zr
aO
2 or Ce
1-c-dZr
cLan
dO
2, wherein Lan is selected from the group consisting of Y, La, Pr, Nd, Sm, Eu, Gd,
Ho, Yb and mixtures thereof.
Figures
[0029]
Figure 1 shows a plot of NOx conversion on the primary Y-axis versus temperature on
the X-axis. Also shown on the secondary Y- axis is NH3 slip as a function of temperature on the x-axis. Data was collected at various space
velocities ranging from 5000 hr-1 to 40,000 hr-1.
Figure 2 shows one embodiment of the invention as used with a fire tube boiler.
Figure 3 shows one embodiment of the invention as used with a water tube boiler.
Figure 4 shows one embodiment of a cassette.
Detailed Description
[0030] Exhaust from industrial and commercial boilers contains nitrogen oxides. It contains
low excess oxygen (typically about 1 to about 10%) and high amounts of water vapor
(about 20%). The nitrogen oxides in the exhaust can be removed by contacting the exhaust
with reducing agents such as, without limitation, ammonia in the presence of a zeolite
based SCR catalyst. The reducing agent reacts with the nitrogen oxides to form nitrogen
and water.
[0031] Water vapor in the exhaust can deactivate the catalyst, lowering the NO
x conversion. However, under the boiler conditions, the zeolite based SCR catalyst
appears to have little or no deactivity from the water vapor. Industrial and commercial
boilers typically have about 20% water vapor.
[0032] The exhaust from industrial and commercial boilers is at low temperature, about 420
- about 590K (about 300-about 600°F). Low temperature activity of the SCR catalyst
is therefore important for industrial and commercial boiler applications. The zeolite
based SCR catalysts according to embodiments of the present invention have good NO
x conversion activity at low temperatures.
[0033] Sulfur although present at low levels in natural gas used as fuel in boilers may
impact the SCR catalyst performance significantly at low temperatures. In 2006 and
2007 natural gas produced by PG&E had a total maximum sulfur level of 15 ppm. Cleaver-Brooks
emissions guide teaches that sulfur present in the exhaust of industrial and commercial
boilers is around 0.34 ppm. The reactor system with a zeolite based SCR catalyst according
to the present invention has good NO
x conversion activity at low temperatures even in the presence of sulfur and large
excess water vapor.
[0034] According to Alcorn (
U. S. Patent 4,912,776), it is believed that the reduction of NO requires the presence of oxygen, while
the reduction· of NO
2 does not. Alcorn also asserts that the reduction of NO
2 is easier to carry out than the reduction of NO. Boiler exhaust has both NO and NO
2 collectively termed as NO
x. The NO component in NO
x can be in the range of about 90-about 100% in the exhaust of industrial and commercial
boilers.
[0035] Alcorn states that the evidence seems to support a two-step process for the SCR process,
where the following reactions occur in parallel:
NO + ½ O
2 →NO
2
6 NO
2 + 8 NH
3 →7 N
2 + 12 H
2O
[0036] It is well-known that SCR catalysts have lower activity for NO
x conversion at high NO/NO
2 ratios than at low ratios. Only about 10% of the NO
x in boiler offgas is NO
2. Low temperature activity of the SCR catalyst at high NO/NO
2 ratios is therefore an important factor. Zeolite based SCR catalysts perform better.
[0037] Without being limited to mechanism, it is believed that the first and second components
along with the zeolite and OSM allow conversion of NO to NO
2 especially at lower temperatures as a first step in the process of reducing NOx.
As more NO
2 is formed, NO/NO
2 ratio reduces when compared to an untreated boiler exhaust. NO
2 is more readily reduced than NO. Therefore lower temperature activity is improved
with the zeolite based SCR catalyst.
[0038] The zeolite based SCR catalysts according to embodiments of the present invention
have higher activity at low temperatures than the catalysts of the prior art. The
zeolite based SCR catalysts of the present invention also have higher hydrothermal
stability than the catalysts of the prior art. Boiler exhaust normally contains a
significant amount of water. Hydrothermal stability is therefore a major factor for
these boiler applications.
Zeolite based SCR Catalyst
[0039] The first component and the second component of the zeolite-base SCR catalysts may
have a synergistic effect on one another. The synergistic effect may help to provide
high NO
x conversion at low temperatures. The synergy between the first component and the second
component may also help to stabilize the zeolite based SCR catalysts toward hydrothermal
aging and sulfur aging.
First Component
[0040] The first component of the zeolite based SCR catalyst is selected from the group
consisting of manganese and/or copper.
[0041] The zeolite based SCR catalyst of the present invention may comprise about 1 to about
20 weight percent of the first component, more preferably about 3 to about 15 weight
percent of the first component, and most preferably about 5 to about 8 weight percent
of the first component, where the weight percent of the first component is calculated
on the basis of the metal.
Second Component
[0042] The second component of the zeolite based SCR catalyst is selected from the group
consisting of cerium, a lanthanide, a mixture of lanthanides, and mixtures thereof,
preferably cerium.
[0043] The zeolite based SCR catalyst of the present invention may comprise about 2 to about
50 weight percent of the second component, more preferably about 5 to about 30 weight
percent of the second component, and most preferably about 5 to about 25 weight percent
of the second component, where the weight percentage of the second component is calculated
on the basis of the metal.
Zeolite
[0044] The zeolite based SCR catalyst comprises at least one zeolite. The zeolite may be
selected from the group consisting of ZSM-5, ZSM- 11, ZSM-12, ZSM-18, ZSM-23, zeolite
beta, a ZSM-type zeolite, a MCM-type zeolite, mordenite, faujasite, ferrierite, and
mixtures thereof, preferably ZSM-5.
[0045] The zeolite may be in the H-form, the Na-form, the ammonium- form, or mixtures thereof,
preferably the H-form of the zeolite.
[0046] The zeolite may also be ion-exchanged, all or in part, with the first component and/or
the second component. The SiO
2/Al
2O
3 ratio of the zeolite may be in a range of about 1 to about 500, more preferably about
10 to about 150, and most preferably about 30 to about 70 (preferably about 40). Although
not wishing to be bound by a theory, it is believed that zeolites having a SiO
2/Al
2O
3 ratio greater than about 10 may be beneficial in enhancing the hydrothermal stability
of the catalysts.
[0047] In an embodiment, all or part of the first component, the second component, or both
the first component and the second component may be impregnated or ion-exchanged into
the zeolite or mixture of zeolites. In an embodiment, part of the first component
and/or the second component may be ion-exchanged into the zeolite or mixture of zeolites,
and part of the first component and/or the second component may be impregnated into
the zeolite or mixture of zeolites.
[0048] The zeolite based SCR catalyst may comprise about 10 to 90 weight percent zeolite,
more preferably about 20 to about 90 weight percent zeolite, and most preferably about
40 to about 80 weight percent zeolite.
Oxygen Storage Material
[0049] The zeolite based SCR catalyst comprises at least one oxygen storage material. Oxygen
storage materials may comprise a cerium-oxide-based material. Oxygen storage materials
can take up oxygen from oxygen-rich feed streams and give up oxygen to oxygen-deficient
feedstreams. The oxygen storage material may also be used as a support for the first
component and/or the second component.
[0050] The total surface area of cerium oxide-based materials may generally decreased when
the cerium oxide-based materials are heated to temperatures of about 1074K (800°C)
or more. One or more metal oxides are added to the cerium oxide-based material to
decrease the degree of sintering of the cerium oxide-based material during exposure
to high temperatures. The oxygen storage material is a cerium oxide-based material
having a composition with the formula Ce
1-aZr
aO
2 or Ce
1-c-dZr
cLan
dO
2, wherein Lan is selected from the group consisting of Y, La, Pr, Nd, Sm, Eu, Gd,
Ho, Yb and mixtures thereof.
[0051] In an embodiment, the oxygen storage material may have a formula of Ce
0.24Zr
0.66La
0.04Y
0.06O
2 (CZLY), Ce
0.24Zr
0.67Ln
0.09O
2 (CZL), Ce
0.68Zr
0.32O
2 (CZO), Ce
0.24Zr
0.67Nd
0.09O
2 (CZN) or Ce
0.6Zr
0.3Nd
0.05Pr
0.05O
2 (CZNP). Other oxygen storage materials may also be suitable.
[0052] The zeolite based SCR catalyst comprises about 10 to about 90 weight percent oxygen
storage material, more preferably about 20 to about 70 weight percent oxygen storage
material, and most preferably about 30 to about 60 weight percent oxygen storage material.
The weight percent of the oxygen storage material is on the basis of the oxides.
[0053] Although not wishing to be limited to a theory, it is believed that the oxygen storage
material may enhance the performance of the zeolite based SCR catalysts by improving
its ability to oxidize NO to NO
2. NO
2 may react more rapidly with ammonia or other reducing agent than does NO. Enhancing
the ability of the catalyst to oxidize NO to NO
2 may therefore improve the activity of the catalyst to catalyze the selective reduction
of NO
x with ammonia.
[0054] The oxygen storage material may also improve the rheology of aqueous slurries for
the optional washcoat (described below) that comprise the oxygen storage material.
Inorganic Oxides
[0055] The zeolite based SCR catalyst may also comprise at least one inorganic oxide selected
from the group consisting of alumina, silica, titania, silica-alumina, zirconia, and
composites, and mixtures thereof, preferably alumina. The inorganic oxides may be
used, for example, as part of a washcoat.
[0056] In an embodiment, the sum of the amount of oxygen storage material and the amount
of inorganic oxide may be an amount as previously given for the oxygen storage material
alone. The other inorganic oxides may be substituted, all or in part, for the oxygen
storage material, although the inorganic oxides may have a different function than
the oxygen storage material. Inorganic oxides may improve the rheology of aqueous
slurries for the optional washcoat and enhance wash-coat adhesion to a substrate,
if the catalyst is to be coated on a monolith.
Zeolite based SCR Catalyst Composition
[0057] The zeolite based SCR catalyst may comprise a substrate. As used herein, a substrate
may be any support structure known in the art for supporting catalysts. In one embodiment
of the present invention, the substrate may be in the form of beads or pellets or
an extrudate. The beads or pellets may be formed from alumina, silica alumina, silica,
titania, mixtures thereof, or any suitable material. In another embodiment of the
present invention, the substrate may be a honeycomb support. The honeycomb support
may be a ceramic honeycomb support or a metal honeycomb support. The ceramic honeycomb
support may be formed, for example, from sillimanite, zirconia, petalite, spodumene,
magnesium silicates, mullite, alumina, cordierite (Mg
2Al
4Si
5O
18), other alumino-silicate materials, silicon carbide, or combinations thereof. Other
ceramic supports may also be suitable.
[0058] If the support is a metal honeycomb support, the metal may be a heat-resistant base
metal alloy, particularly an alloy in which iron is a substantial or major component.
The surface of the metal support may be oxidized at elevated temperatures above about
1000° C to improve the corrosion resistance of the alloy by forming an oxide layer
on the surface of the alloy. The oxide layer on the surface of the alloy may also
enhance the adherence of a washcoat to the surface of the monolith support. Preferably,
the substrate, either metallic or ceramic, offer a three-dimensional support structure.
[0059] In one embodiment of the present invention, the substrate may be a monolithic carrier
having a plurality of fine, parallel flow passages extending through the monolith.
The passages can be of any suitable cross-sectional shapes and sizes. The passages
may be, for example, trapezoidal, rectangular, square, sinusoidal, hexagonal, oval,
or circular, although other shapes are also suitable. The monolith may contain from
about 9 to about 1200 or more gas inlet openings or passages per square inch of cross
section, although fewer passages may be used.
[0060] The substrate can also be any suitable filter for particulates. Some suitable forms
of substrates may include woven filters, particularly woven ceramic fiber filters,
wire meshes, disk filters, ceramic honeycomb monoliths, ceramic or metallic foams,
wall flow filters, and other suitable filters. Wall flow filters are similar to honeycomb
substrates for automobile exhaust catalysts. They may differ from the honeycomb substrates
that may be used to form normal automobile exhaust catalysts in that the channels
of the wall flow filter may be alternately plugged at an inlet and an outlet so that
the exhaust is forced to flow through the porous walls of the wall flow filter while
traveling from the inlet to the outlet of the wall flow filter.
Washcoat
[0061] In an embodiment, at least a portion of the catalyst of the present invention may
be placed on the substrate in the form of a washcoat.
[0062] In an embodiment, a washcoat may be formed on the substrate by suspending the zeolite
and/or OSM in water to form an aqueous slurry and placing (placing includes but is
not limited to depositing, adhering, curing, dipping, applying, and spraying) the
aqueous slurry onto the substrate as a washcoat. In an another embodiment, the washcoat
may further comprise at least one inorganic oxide selected from the group consisting
of alumina, silica, titania, silica-alumina, zirconia and solid solutions, and combinations
thereof.
[0063] In other embodiments, other components such as salts of the first and/or the second
components may optionally be added to the aqueous slurry. Other components such as
acid or base solutions or various salts or organic compounds may be added to the aqueous
slurry to adjust the rheology of the slurry. Some examples of compounds that can be
used to adjust the rheology include, but are not limited to ammonium hydroxide, aluminum
hydroxide, acetic acid, citric acid, tetraethylammonium hydroxide, other tetralkylammonium
salts, ammonium acetate, ammonium citrate, glycerol, commercial polymers such as polyethylene
glycol, and other suitable polymers.
[0064] In an embodiment, the first component, the second component, or both the first component
and the second component may be added to the aqueous slurry as oxides or other compounds,
for example nitrates, acetates or other salts and/or mixture of thereof. The slurry
may be placed onto the substrate in any suitable manner. If the substrate is a monolithic
carrier with parallel flow passages, the washcoat may be formed on the walls of the
passages. Gas flowing through the flow passages may contact the washcoat on the walls
of the passages as well as materials that are supported on the washcoat.
[0065] It is believed that the oxygen storage material may enhance the rheology of the washcoat
slurry. The enhanced rheology of the washcoat slurry that may be due to the presence
of the oxygen storage material may enhance the adhesion of the washcoat slurry to
the substrate.
[0066] In an embodiment, a washcoat may be formed by slurry depositing the zeolite and the
oxygen storage material onto the substrate. The washcoat may also comprise at least
one inorganic oxide selected from the group consisting of alumina, silica, titania,
silica-alumina, zirconia and solid solutions, composites, and mixtures thereof. A
solution comprising water-soluble precursor salts of the first component and/or the
second component may be impregnated and/or ion-exchanged into the washcoat after the
washcoat is placed on the substrate. In an alternative embodiment, salts of the first
and/or the second component may be added to the aqueous slurry for the washcoat. In
yet another embodiment, at least one of the first component, and the second component,
may be added to the aqueous slurry for the washcoat as oxides.
[0067] In an embodiment, the substrate, the washcoat, and the impregnated or ion-exchanged
solution (comprising water-soluble precursor salts of the first component and/or the
second component) may be calcined to form the catalyst composition before or after
the washcoat and/or the solution are added to the substrate. In an embodiment, the
washcoat and the impregnated or ion-exchanged solution may be dried before calcining.
Catalyst Cassette and Zeolite based SCR Catalyst Reactor
[0068] In a preferred embodiment, the honeycomb catalysts (preferably coated and calcined)
can be formed into a catalyst cassette. In one embodiment as shown in Figure 4, the
individual honeycomb catalysts that form the catalyst cassette (8) are called as catalysts
elements which are held in spaces (9) within the cassette. The dimensions of the catalyst
cassette can be from one or more catalyst elements depending on the boiler exhaust
requirements and reactor space requirements. Several cassettes may be used adjacent
to or stacked on each other within a layer. The layers may be modular so that multiple
layers may be used in the reactor but one or more layer can be removed and/or replaced
at a time. The cassettes may be modular as well so multiple cassettes may be used
in each layer but one or more cassette can be removed and/or replaced at a time. In
some embodiments, the removal, replacement and/or repair of one or more cassettes
and/or one or more layers may be conducted while the boiler is operating. The reactor
may be built so that one or more of the cassettes and/or layers may be accessed, removed,
added and/or replaced without dismantling the entire reactor (e.g. as modular units).
[0069] The depth of the zeolite based SCR catalyst layer can be anywhere from 0.076 to 3.28
m (3 to 120") depending on performance and pressure drop requirements. More preferably
the depth of zeolite based SCR catalyst can be from 0.076m (3") to 0.91m (36"). One
or more layers may be used in the reactor in series or adjacent to each other. If
needed, the SCR catalyst cassettes can be easily removed and stored at site when the
boiler is not operational. The cassettes comprise one or more catalyst elements that
are either 0.3m (12") or 0.15m (6") or 0.076m (3") thick. Typical cross-section of
each catalyst element can be but not limited to 0.15m (6") x 0.15m (6"). The catalyst
element is typically a coated honeycomb substrate with anywhere from 0.0065-0.52 cells
per square meter (10-800 cells per square inch). The catalyst element can be extruded
into any shape. For example, the catalyst can be extruded into a honeycomb of suitable
size and cassettes may be formed from such an extruded honeycomb. Beads or pellets
can be made from such a zeolite based SCR catalyst and can be packed to form a zeolite
SCR catalyst into a cassette.
[0070] In one embodiment, the reactor is arranged such that the flow of the exhaust is substantially
perpendicular to the catalyst cassettes. The reactor can be located either horizontally
or vertically or in any direction as dictated by the application needs. In an embodiment,
the reactor can be located in the boiler stack or any place in the path of the exhaust.
[0071] In an example, a catalyst cassette dimension is 0.3m (1 feet) wide x 2.4m (8 feet)
long x 0.15m (0.5 feet) deep. The catalyst cassettes can be contained in a container
that forms the zeolite based SCR catalyst reactor. A zeolite based SCR catalyst reactor
can have several catalyst cassettes. The amount of the catalyst is dictated by the
dimensions of the zeolite based SCR reactor that is required to reduce emissions from
the boiler exhaust. Although the number of catalyst cassettes are not limited for
any boiler, for a typical boiler application, preferably 2 or 3 catalyst cassettes
are used. This modularized approach will allow ease of installation and ease of replacement.
Also, the catalyst cassettes can be removed easily and stored appropriately when the
boiler is not under operation. Additional layers of catalyst cassettes can be added
in the future, as catalyst activity decreases over time, to extend the lifetime of
the performance of the existing system, or to lower NO
x emissions to meet new regulatory requirements.
[0072] In one embodiment shown in Figure 2, the zeolite-based SCR catalyst (1) is used with
a fire tube boiler (2). The reactor (6) which contains the zeolite-based SCR catalyst
(1) is located in the stack (4) and the reducing agent is injected into the exhaust
(5) before the exhaust reaches the zeolite-based SCR catalyst. The zeolite-based SCR
catalyst is formed into cassettes (3) and multiple cassettes and/or multiple layers
may be used in a single reactor. This helps so that if one cassette needs to be replaced,
the entire catalyst does not have to be removed. The reactor may be built so that
one or more of the cassettes and/or layers may be accessed, removed, added and/or
replaced without dismantling the entire reactor (e.g. as modular units).
[0073] In another embodiment shown in Figure 3, the zeolite-based SCR catalyst (1) is used
with a water tube boiler (7). The reactor (6) which contains the zeolite-based SCR
catalyst (1) is located after the boiler (7) but before the stack (4) and the reducing
agent is injected into the exhaust (5) before the exhaust reaches the zeolite-based
SCR catalyst. The zeolite-based SCR catalyst is formed into cassettes (3) and multiple
cassettes and/or multiple layers may be used in a single reactor. This helps so that
if one cassette needs to be replaced, the entire catalyst does not have to be removed.
Method for Removing NOx
[0074] In an embodiment, the exhaust from a boiler contacts the zeolite based SCR catalyst
in the presence of a reducing agent comprising ammonia sufficient to reduce the NO
x that is contained in the boiler exhaust to the desired level. The reducing agent
may be introduced into the exhaust before the exhaust contacts the zeolite based SCR
catalyst or at the time that the exhaust contacts the zeolite based SCR catalyst.
Static mixers, flow deflecting vanes and/or other devices may be used to mix the reducing
agent with the exhaust before or when it reaches the zeolite based SCR catalyst reactor.
The zeolite based SCR catalyst can be located anywhere from the boiler exit to the
end of the boiler stack. In an embodiment, the zeolite based SCR catalyst is located
in the boiler stack. In another embodiment, the zeolite based SCR catalyst is located
in an economizer. In another embodiment, the zeolite based SCR catalyst is located
after the economizer. In another embodiment, the zeolite based SCR catalyst is located
before the boiler economizer. The boiler exit breach, the economizer, the stack, and
any connecting duct work, or any of combinations thereof can be modified to locate
the zeolite based SCR catalyst and more than one location may be used in a single
embodiment.
[0075] In an embodiment, the ammonia/NO
x mole ratio may be in a range of about 0.01 to about 2.5, more preferably in a range
of about 0.7 to about 2, and most preferably in a range of about 0.8 to about 1.2.
Low ammonia /NO
x ratios may generally be preferred in order to minimize excess ammonia in the exhaust.
Excess ammonia in the exhaust may be undesirable due to health or odor issues.
[0076] The space velocity of the exhaust and the reducing agent passing through the zeolite
based SCR catalyst may be in a range of about 1,000 hr
-1 to about 180,000 hr
-1, more preferably in a range of about 1,000 hr
-1 to about 90,000 hr
-1, and most preferably in a range of about 1,000 hr
-1 to about 60,000 hr
-1.
[0077] The exhaust and reducing agent may be contacted with the zeolite based SCR catalyst
at about 410K (140° C) to about 970K (700° C), more preferably at about 420K (150°
C) to about 870K (600° C), and most preferably at about 420K (150°C) to about 770K
(500°C).
[0078] If the temperature of the exhaust is lower than about 420K (150° C), the reduction
of the nitrogen oxides may be low. At temperatures greater than about 770K (500° C),
the ammonia may be oxidized. If the ammonia is oxidized, there may be insufficient
ammonia reducing agent in the exhaust to reduce the nitrogen oxides.
[0079] If excess ammonia is present in the exhaust, at least a portion of the excess ammonia
may be oxidized to nitrogen by the catalyst according to embodiments of the present
invention.
[0080] The following examples are intended to illustrate, but not to limit, the scope of
the invention. It is to be understood that other procedures known to those skilled
in the art may alternatively be used.
EXAMPLE 1
NOx conversion and NH3 slip at various space velocities
[0081] A slurry was made comprising of 50% HZSM-5 zeolite and 50% oxygen storage material
with water. This formed the washcoat. This washcoat was coated on a 210 cpsi ceramic
honeycomb substrate to achieve a loading of 0.2 g/m
3 (200 grams per liter) of the washcoat on the ceramic honeycomb 210 cpsi substrate.
The channels of the wet washcoated substrate were cleared by blowing air using an
air knife. The washcoated substrate was fired at 823K (550°C) for 4 hours to get a
calcined washcoated substrate. A solution mixture of cerium nitrate, manganese nitrate
and copper nitrate was made and impregnated on to the washcoated honeycomb substrate.
The solution of cerium nitrate, manganese nitrate and copper nitrate was made such
that it will result in a cerium loading of 0.02 g/m
3 (20 grams per liter), manganese loading of 0.0125 g/m
3 (12.5 grams per liter) and copper loading of 0.00525 g/m
3 (5.25 grams per liter) on the final impregnated honeycomb catalyst. This catalyst
was tested at boiler exhaust conditions using a stream of 40 ppm NO
x, 40 ppm NH
3, 50 ppm CO, 10% CO
2, 20% H
2O and balance N
2. The results in terms of NO
x conversion and NH
3 slip are presented in Table 2 as a function of space velocity at 450K (350°F (176.6°C)).
Table 2 NOx conversion and NH3 slip using zeolite based SCR catalyst at boiler exhaust conditions at 450K (350°F).
Space Velocity, hr-1 |
NOx conversion, % |
NH3 slip, PPM |
5000 |
100 |
0 |
10,000 |
97 |
0 |
20,000 |
88.8 |
0 |
30,000 |
86.4 |
0 |
40,000 |
83.2 |
0.4 |
[0082] As shown in Figure 1 and Table 2, NOx conversions in excess of 90% were achieved
using this catalyst when the temperature is above 450K (350°F) and when the space
velocity was below 15,000 hr-1. As the space velocity increased from 15,000 hr-1 to
40,000 hr-1, NOx conversion decreased from more than 90% to more than 75%. An ammonia
slip of greater than 0.2 ppm was observed only when the temperature was below 450K
(350°F) and when the space velocity was above or equal to 40,000 hr-1. Most industrial
and commercial boilers have exhaust temperatures above 450K (350°F).
[0083] Table 3 shows the results of activity tests performed using the catalyst described
in Example 1 at boiler exhaust conditions. The stream used to test the catalyst activity
was again 40 ppm NO
x, 40 ppm NH
3, 50 ppm CO, 10% CO
2, 20% H
2O and balance N
2. A "Pass/Fail" criteria was used to determine the catalyst activity. A "Pass" to
the activity test was assigned when the observed NO
x conversion and NH
3 slip after the catalyst were less than 5 ppm NOx and less than 5 ppm NH
3. The "Pass" criteria is such that both NO
x and NH
3 slip should independently be less than 5 ppm. In other words, if the activity test
resulted in 6 ppm NO
x and 2 ppm NH
3 slip, or if in an another test NO
x was 2 ppm, however NH
3 slip was 6 ppm, the test result in both cases is assigned as "fail".
Table 3: Pass or fail result on activity test based on activity test criteria Activity
test criteria: Post catalyst NOx and NH3 should be less than 5 ppm.
Space Velocity, hr-1 |
Temperature, K(°F) |
|
408 (275) |
422 (300) |
436 (325) |
450 (350) |
464(375) |
478(400) |
5,000 |
Fail |
Pass |
Pass |
Pass |
Pass |
Pass |
10,000 |
Fail |
Fail |
Pass |
Pass |
Pass |
Pass |
15,000 |
Fail |
Fail |
Fail |
Pass |
Pass |
Pass |
20,000 |
Fail |
Fail |
Fail |
Pass |
Pass |
Pass |
30,000, |
Fail |
Fail |
Fail |
Fail |
Pass |
Pass |
40,000 |
Fail |
Fail |
Fail |
Fail |
Fail |
Pass |
Example 2
NOx conversion and NH3 slip at various temperatures and space velocities on sulfur aged catalyst.
[0084] Boiler exhaust may contain sulfur compounds primarily as SO
2. The amount of SO
2 in a boiler exhaust depends on the type of fuel being burnt in the boiler. Pipeline
natural gas is a common fuel for industrial and commercial boilers. Natural gas suppliers
such as PG&E provide information of the amount of sulfur present in natural gas. In
the years or 2006 and 2007, PG&E provided pipeline natural gas that had total maximum
sulfur anywhere from 3 ppm to 15 ppm. Cleaver Brooks, a manufacturer of industrial
and commercial boilers, published emissions reference guide that mentions that such
a sulfur containing natural gas when burnt in industrial and commercial boilers will
result in a SO
2 concentration of about 0.34 ppm in the boiler exhaust. Tests were performed using
the catalyst described in example 1 and subjecting the catalyst to a sulfur aging
using a stream comprising of 1 ppm of SO
2 in addition to 20% H
2O, 40 ppm NO
x, 40 ppm NH
3, 50 ppm CO, 10% CO
2 and balance N
2. Activity data was collected at various space velocities and temperatures after subjecting
the catalyst to the above mentioned sulfur aging for 100 hours. Table 4 describes
the results obtained after such sulfur aging at 473K (200°C (392°F)).
Table 4. Performance of Zeolite based SCR catalyst after 100 hours of time-on-stream
aging with 1 ppm S02.
Space Velocity, hr-1 |
Temperature, K(°F) |
Post catalyst NOx and NH3 slip in ppm |
NOx,ppm |
NH3 slip, ppm |
10,000 |
478(400) |
5 |
5 |
30,000 |
533(500) |
3 |
3 |
30,000 |
589(600) |
1 |
1 |
[0085] The zeolite based SCR catalysts have higher activity for selective reduction of NO
x with ammonia than the catalysts of the prior art. Further, the catalysts according
to embodiments of the present invention have high NO
x conversion activity at various temperatures even after subjecting the catalyst with
time-on-stream SO
2 aging.
[0086] The zeolite based SCR catalysts although suitable for industrial and commercial boilers,
may have applications to other gas streams that contain NO
x, particularly to exhaust streams with either low temperatures or when there is a
vast temperature changes during the operation of the application. Some examples of
such applications include, but are not limited to, exhaust from diesel engine powered
vehicles, exhaust from gas turbines, exhaust from diesel generators, exhaust from
electrical generation plants, exhaust from chemical plants, and other suitable applications.